Like most things in football, the offside rule is pretty simple - but Asier Del Horno's disallowed goal for Chelsea against Birmingham on Saturday has re-drawn the grey areas once more.

Although Del Horno scored from an onside position, fellow defender Ricardo Carvalho attempted to head the ball, having come back from an offside position.

An amendment to the rule was introduced at the start of the 2003/04 season, which allows a player to be in an offside position provided he or she is not "actively involved in play".

It was designed to promote attacking football, but different interpretations of what constitutes "active play" have led some to suggest it is open to abuse.

Fifa, world football's governing body, has clarified when a player is to be regarded as "actively involved in play":

"Interfering with play means playing or touching the ball passed or touched by a team-mate."

The changes apply to all levels of football and Manchester United's Ruud van Nistelrooy often exploits this by standing in an offside position as a free-kick is about to be taken.

He is taking advantage of the rule that, if the ball doesn't come to him, then he is not "active" and therefore onside, which is the argument Chelsea manager Jose Mourinho used for Ricardo Carvalho's attempt to score.

However, a player doesn't necessarily have to touch the ball to influence the play. They are still offside if they are judged to be:

Interfering with an opponent - If an attacker interferes with an opponent by either preventing them from playing or being able to play the ball, then they are offside. This could be done by blocking the goalkeeper, or obstructing their line of vision.

Gaining an advantage - If the ball is played into the penalty area and rebounds off either a post, the crossbar or an opposing defender, then the attacker is offside as they have gained an advantage by being in that position.

So was Carvalho obstructing the line of vision of the goalkeeper? The referee's assistant obviously thought so.

OFFSIDE ESSENTIALS

Got your head around the new rules? Here's our guide to the basics.

A player is in an offside position if, when the ball is played by a team-mate, they are nearer to the opposition's goal line than both the ball and the second last opponent.

Clear so far? There are a few more things to remember.

You can't be offside if:

You receive the ball directly from a goal kick, a throw-in or a corner

you are in your own half of the pitch

you are level with the second last or last two opponents

you are level with or behind the team-mate who plays you the ball

you are not actively involved in play, as explained above

For any offside offence, the referee awards an indirect free-kick to the opposing team, to be taken from the place where the infringement occurred.

Still confused? We've put together a number of offside examples to help explain the rules.